1. Prior Art
Friction welding has a long and successful history, and has proven effective with a substantial number of metals.
In the most general terms, friction welding is performed by rubbing contact between mating surfaces of two pieces of metal under sufficient pressure that the level of heat generated by the resulting friction elevates the temperature of the contact areas to welding temperature. Once the temperature level is sufficient, the rubbing motion is halted, and the two pieces are pressed together to form a weld at the heated contact area.
In circumstances to which friction welding is applicable, excellent weld joints are obtained. Indeed, friction welding is often superior to other techniques of welding, particularly when unlike metals are joined. A wide diversity of metals may be joined to themselves and to other metals by friction welding. In addition, no extraneous materials are employed, and a well performed friction weld provides strength as high as that of the substrate metal.
The rubbing contact of friction welding has been attained by providing relative rotation or relative reciprocal linear oscillation of the workpieces for the time and at the pressure required to attain effective welding temperatures. Such relative motions are ordinarily obtained by holding one part stationary while the other is rotated or oscillated with the surfaces to be welded pressed together in frictional contact. In a few cases, motion of both parts has been employed, by counter-rotation or counter-oscillation of parts. In a few systems, the superimposition of other motions on the base rotary or linear oscillatory motion have been suggested. The limitations of friction welding are dictated by the consequences of the relative motions employed. Of course the system is constrained by the requirement for the shapes and particularly the contact areas, which must be compatible with the relative motions required. In addition, the formation of a weld bond is rapid when the relative motion is stopped, typically within less than 1 second, and often as little as 0.2 seconds or less, and the precision of the alignment of the pieces is often limited by the inertia of the relative motion mechanisms and of the equipment employed to attain such motion. The inertia of the system is an important factor, because the initial bonding of the parts is extremely rapid and often occurs before the alignment of the parts to be joined can be attained. Alignment and bonding are performed when the drive mechanism and the relative motions are stopped. It is generally impossible to employ friction welding when high precision is required. If temperatures become too high, the metals of the parts may seize and bond with no alignment, and potentially causing damage to the part or to the welding mechanism.
The difficulties of friction welding with rotary motion are made considerably more complex if the rotating part does not have symmetry about the axis of rotation. While slight asymmetry can be accommodated in some systems, the rotary imbalance of parts further degrades the accuracy and precision of the operation, and limits the parts with which the techniques can be employed. Substantial asymmetry precludes the use of rotary friction welding.
In addition, friction welding has heretofore been limited to joints which are at least generally circular or linear in the contact area between the parts to be joined, and have not been employed where more complex shapes are to be joined.
2. Needs in the Art
There are a number of industrial manufacturing operations and fabrications which would be highly desirable candidates for the employment of friction welding if the precision of alignment required could reliably be attained. A representative example of such fabrication is the attachment of turbine blades to turbine rotors and stators in the manufacture of turbine engines for jet engines.
At the present time, turbine blade-rotor structures and turbine blade-stator are commonly made by one of two techniques. Either the piece is machined from a solid monolithic block, or the blades are provided with a keying structure, commonly referred to as a Christmas tree, inserted into a mating keyway slot.
Both techniques have disadvantages. The monolith approach requires exceedingly complex and expensive machining techniques, and does not admit of repair or replacement of individual blades. The Christmas tree approach affords greater flexibility but requires considerable manufacturing cost and adds to the weight of the component to provide the mass and bulk required to achieve the necessary strength under the extremes of operation of a turbine engine.
It would be highly desirable, both for ease and economy of manufacture, and for rework or repair of the parts, if the blades could be welded to the rotor or stator. Welding has not heretofore been employed in the manufacture or repair of such structures, however. Friction welding is not employed because of the difficulties in attaining adequate, reliable and certain alignment of the parts. Other welding techniques are not used because of the limited strength of weld joints and related difficulties.
Techniques for use with asymmetric parts having non-linear joints and greater flexibility in joint design are also needed in the art.
Orbital motion has been proposed in the art. See for example:
Craine, et al., "Frictional Heat Generated in the Early Stages of an Orbital Friction Welding Process," Wear, 114, pp. 355-365, Elsevier, Den Hague, 1987, propose combining rotation and orbital motions, where the parts to be joined are rotated in mating configuration, in the same direction and at the same speed, and where the axis of rotation is moved in an orbital path to produce friction until welding temperature is attained, whereupon the axes of rotation are brought back into coincidence, so that the relative motion between the parts is terminated. The Craine, et al., strategy does not eliminate the requirement for rotating parts, resulting in complex and difficult to control equipment.
Searle I, "Orbital Friction Welding: Theory into Practice," Design Engineering, September, 1971, pp. 48-50, discloses orbital welding where one element is orbited while the other is held stationary. Such operations are difficult to control because of the difficulty of controlling the position and relative alignment of the pieces while braking the orbital motion and alignment inaccuracies result.
Searle II, "Friction Welding Moves into a New Orbit," Metalworking Prod., Jul. 1971, pp. 72-74, is cumulative to Searle I, supra.
"Friction Welding Lines Up More Jobs," Machinery and Production Engineering, May 2, 1979, PP. 41-43, inter alia, discusses the limitations of the procedure disclosed by Searle I and II, supra.
MacLaughlin, et al. U.S. Pat. No. 4,462,849, discloses and claims axial oscillatory welding. No orbital component is disclosed.
MacLaughlin, et al. U.S. Pat. No. 4,515,651, is a Continuation in part of MacLaughlin U.S. Pat. No. 4,462,849, supra and is cumulative in import.
Nicholas, "Fabricating by Friction," Engineering, Apr., 1985, pp. 254-256, discusses the procedure of the type disclosed in Craine, et al., supra, and is cumulative in substance.
Watson, "Welding Plastics for the Automotive Industry," SAE Technical Paper Series, No. 860581, Feb. 24-28, 1986, discloses a variety of plastic welding techniques in a survey, including orbital friction welding of thermoplastic polymers.
"Linear Friction Enables Non-Circular Metal Welding," Eureka, Mar., 1989, pp. 75-79, discloses linear oscillatory friction welding wherein the parts are periodically realigned in differing orientation relative to one another to attain better and particularly more uniform surface heating. A rotary motion coupled with lateral translation is also discussed.